Activist investor Nelson Peltz won a board seat at Mondelez International Inc. but relinquished his efforts to merge the food company with the snacks business of PepsiCo Inc.

The board seat marks a qualified victory for Mr. Peltz, who has pushed for months for Mondelez to improve its profit margins. His concession on PepsiCo represents an acknowledgment that his larger goal to merge the businesses into a global snacking powerhouse may be unattainable, but Mr. Peltz escalated his criticism of management. He said he will...